--- a/src/core/ngx_string.h
+++ b/src/core/ngx_string.h
@@ -140,7 +140,11 @@ u_char *ngx_cpystrn(u_char *dst, u_char 
 u_char *ngx_pstrdup(ngx_pool_t *pool, ngx_str_t *src);
 u_char * ngx_cdecl ngx_sprintf(u_char *buf, const char *fmt, ...);
 u_char * ngx_cdecl ngx_snprintf(u_char *buf, size_t max, const char *fmt, ...);
-u_char *ngx_vsnprintf(u_char *buf, size_t max, const char *fmt, va_list args);
+u_char * ngx_cdecl ngx_slprintf(u_char *buf, u_char *last, const char *fmt,
+    ...);
+u_char *ngx_vslprintf(u_char *buf, u_char *last, const char *fmt, va_list args);
+#define ngx_vsnprintf(buf, max, fmt, args)                                   \
+    ngx_vslprintf(buf, buf + (max), fmt, args)
 
 ngx_int_t ngx_strcasecmp(u_char *s1, u_char *s2);
 ngx_int_t ngx_strncasecmp(u_char *s1, u_char *s2, size_t n);